This study will evaluate the safety and effectiveness of an antibiotic called Fansidar on autoimmune lymphoproliferative syndrome (ALPS). Patients with ALPS have enlarged lymph glands, spleen and/or liver, abnormal blood cell counts and overactive immune function. Current treatments are aimed at suppressing the immune system and improving symptoms, such as anemia (low red blood cell count) and low white blood cell and platelet counts. These treatments, however, are only partially effective and may have complications. Fansidar is a combination of two drugs, sulfadoxine and pyrimethamine, that is used to treat or prevent parasitic infections such as malaria. Recently a child with ALPS who was treated with Fansidar for a different illness had a marked shrinkage of the lymph organs. This study will examine whether Fansidar can shrink the lymph glands or spleen in patients with ALPS.